Did you actually read the manga properly? In no place did it say they killed their best friend to unlock MS, just that they trained:
So its possible that they simply experienced another loss along the way or unlocked it after reaching a certain power coupled with loosing their brothers in the past.
In addition, you could say Madara and Hashirama and Izuna and Tobirama simply weren't best friends.
They had members of their respective clans that they were best friends with. Keep in mind also that they did not unlock EMS until their teenage years, so after this event we saw just now, they were no longer able to associate with eachother, so they likely made new and better friends later.
You must be registered for see links
You must be registered for see links
So its possible that they simply experienced another loss along the way or unlocked it after reaching a certain power coupled with loosing their brothers in the past.
In addition, you could say Madara and Hashirama and Izuna and Tobirama simply weren't best friends.
They had members of their respective clans that they were best friends with. Keep in mind also that they did not unlock EMS until their teenage years, so after this event we saw just now, they were no longer able to associate with eachother, so they likely made new and better friends later.